The present invention relates generally to a measuring apparatus that measures the optical performance of an optical element, and more particularly to an exposure apparatus mounted with a measuring apparatus that measures a wave front aberration of a projection optical system that transfers a reticle pattern onto an object to be exposed.
A projection exposure apparatus has conventionally been used to transfer a circuit pattern of a reticle (or a mask) onto an object to be exposed in manufacturing such devices as semiconductor devices, e.g., an IC and a LSI, an image pickup devices, e.g., a CCD, display devices, e.g. a liquid crystal panel, and magnetic heads, in the photolithography process. Since this exposure apparatus needs to precisely transfer a reticle pattern onto a wafer at a predetermined magnification, it is important to use a precise projection optical system having good imaging performance and reduced aberration. In general, a value of a root mean square (“RMS”) that indicates the precision of the optical system should be λ/14 or smaller in view of the Mareshal criterion, where λ is a wavelength of a light source.
A catoptric optical system that includes n mirrors is used for an exposure apparatus that uses the extreme ultraviolet (“EUV”) light with a wavelength of λ of about 13.5 nm (“EUV exposure apparatus” hereinafter) and attempts to meet the recent demand for finer processing to the semiconductor device. Each mirror requires a shaping precision of λ/(28√n), and a six-mirror optical system needs a surface processing precision of about 0.2 nm RMS.
A conventional surface-precision measuring apparatus cannot measure such a highly precise surface shape due to its insufficient measuring precision. Accordingly, a measuring apparatus with such a high measuring precision as about 0.1 nm RMS has been proposed which utilizes a point diffraction interferometer (“PDI”) that has a pinhole for generating an ideal spherical wave, and a line diffraction interferometer (“LDI”) that has a slit for generating an ideal cylindrical or elliptical wave. See, for example, Katsuhiko Murakami, “O plus E”, New Technology Communications, Inc., 2004 January, Vol. 26, No. 1, pp. 43-47.
In the measuring apparatus that uses the PDI and LDI for interference between a test wave front and a reference wave front generated from a fine aperture, such as the pinhole and slit, an error between the reference wave front and an ideal spherical or cylindrical wave, which error is referred to as a reference wave front deviation, affects a measuring error. The reference wave front deviation is caused by an optical axis offset or an offset between an optical axis of the incident light and an optical axis or center of the pinhole or slit. The optical axis offset causes the incident light to be shielded by the edge of the pinhole or slit, and disturbs the exited reference wave front. In addition, although a sufficiently small and thin pinhole or slit in a perfectly light-shielding member generates an ideal spherical or cylindrical wave, an actual pinhole or slit has a finite thickness and the generated wave front has a reference wave front deviation.
Therefore, it is necessary for the reduced reference wave front deviation to make a size of the aperture as small as possible and precisely align the center of the aperture with the optical axis of the incident light. Regarding the above influence, it is reported that a measuring error problematically increases when the wave front exited from the pinhole is calculated by changing the beam shift and the wave front aberration, and used as a reference wave front. See, for example, Y. Sekine, A. Suzuki, M. Hasegawa, H. Kondo, M. Ishii, J. Kawakami, T. Oshino, K. sugisaki, Y. Zhu, K. Otaki, Z. Liu, “Wave-front errors of reference spherical waves in high-numerical aperture point diffraction interferometers” J. Vac. Sci. Technol. B22(1) 2004.
The conventional interferometry uses a wavelength of the visible light, and does not require a high measuring precision, neglecting the influence of the reference wave front deviation on the measuring precision.
However, the projection optical system in the EUV exposure apparatus requires a highly precise measurement of the wave front aberration, and the influence of the reference wave front deviation on the measuring precision does not become negligible. In addition, the light having a small wavelength, such as the EUV light, leaks into a member that has a fine aperture, such as a light-shielding membrane, and the light leaking into the fine aperture disturbs the reference wave front similar to offsetting the optical axis. As the member that has the fine aperture is made thicker, the influence of the leakage into the fine aperture on the EUV light reduces. However, this scheme is contrary to the demand for a thinner aperture for an ideal spherical or cylindrical wave.
On the other hand, as disclosed in the above second reference, the influence on the measuring precision is non-negligible for the reference wave front emitted from the conventional fine aperture, since the beam shift etc. increase the wave front deviation. Therefore, a finer aperture that generates a reference wave front closer to the ideal spherical or cylindrical wave is preferable for the improved measuring precision.
Thus, the conventional measuring apparatus cannot precisely measure the aberration suitable for a highly precise optical system. In other words, the conventional measuring apparatus does not meet the measuring precision required for the highly precise optical system.